> On the SIMS server that I run, I'm also the recipient of some of the
> eMail addresses.
>
> I have two addresses MembershipData@ClanMoffat.org and
> Membership@ClanMoffat.org that were used by two different people for
> handling membership matters, but now I am doing both job functions,
> and I'd like all the mail for MembershipData to be delivered to
> Membership.
>
> So in the Router I put
>
> <MembershipData> = Membership
>
> However when my mail client (Mail.app under Mac OS X) retrieves these
> messages, they still show as being "To" MembershipData, so when I
> reply the reply goes out from MemberhshipData.

This is the expected behavior. The headers of a message (To, From, Subject,
etc.) are part of the message's data. SIMS (being an RFC compliant MTA)
does not alter message data. In your case, even though messages are
delivered to the 'membership' user account, they may have been sent to the
address <membershipdata@clanmoffat.org> with the envelope recipient
<membershipdata@clanmoffat.org>. In many cases it would be harmful and|or
confusing to change either the To header or the envelope recipient and it's
not an MTA's job to make that determination.

> Can SIMS handle doing whatever is necessary so that these messages
> show that they actually are for Membership by changing the To address
> so that a reply goes out from the Membership address?

No. That is more properly a job for your mail client (MUA).

> Or do I have to remember to make sure I reply from the correct
> address each time I create a reply.

Yes. Depending on the mail client you use, you might be able to automate
it.